Corrosion resistant metastable austenitic steel

ABSTRACT

A STEEL AND A PROCESS FOR IMPARTING TO SUCH STEEL A COMBINATION OF HIGH STRENGTH, HIGH UNIFORM ELONGATION, HIGH TOUGHNESS AND CORROSION RESISTANCE. THE PROCESS CONSISTS OF SUBJECTING SINGLE PHASE, AUSTENITIC STEEL WHICH HAS AN MS BELOW AMBIENT TEMPERATURE, A TOTAL CARBON PLUS NITROGEN CONTENT OF FROM ABOUT 0.15% TO ABOUT 0.5%, A CHROMIUM CONTENT OF FROM ABOUT 11% TO ABOUT 18%, AND AT LEAST 0.5% OF AT LEAST ONE ALLOYING ELEMENT SELECTED FROM THE GROUP CONSISTING OF MOLYBDENUM, MANGANESE, VANADIUM, NIOBIUM, TANTALUM AND TUNGSTEN PREFERABLY MOLYBDENUM, TO DEFORMATION AT A TEMPERATURE ABOVE ABOUT 400*F. BUT BELOW THE RECRYSTALLIZATION TEMPERATURE OF THE STEEL.

United States Patent O Int. Cl. C21d 7/14 US. Cl. 14812 9 ClaimsABSTRACT OF THE DISCLOSURE A steel and a process for imparting to suchsteel a combination of high strength, high uniform elongation, hightoughness and corrosion resistance. The process consists of subjectingsingle phase, austenitic steel which has an M below ambient temperature,a total carbon plus nitrogen content of from about 0.15% to about 0.5%,a chromium content of from about 11% to about 18%, and at least 0.5 ofat least one alloying element selected from the group consisting ofmolybdenum, manganese, vanadium, niobium, tantalum and tungstenpreferably molybdenum, to deformation at a temperature above about 400F. but below the recrystallization temperature of the steel.

BACKGROUND OF THE INVENTION The invention described herein was made inthe course of, or under, Contract No. W-7405-Eng-48 with the UnitedStates Atomic Energy Commission.

The invention relates to high strength, high elongation steel, and moreparticularly to a corrosion resistant high strength, high toughness,high uniform elongation steel, and to a process for making same.

High strength, high elongation steel and a process for making same isdisclosed and claimed in US. Pat. No. 3,488,231, assigned to the sameassignee. However, this prior steel is not corrosion resistant, thuslimiting the applications for high strength, high elongation steels.

The composition of the present inventive corrosion resistance steel isgenerally similar to that of the steel disclosed in the above referencedpatent, and is produced in a generally similar manner, but provides asubstantial improvement over this prior high strength, high elongationsteel.

SUMMARY OF THE INVENTION The starting material of the inventive process,like that of the process of the above referenced patent, requires an Mbelow ambient temperature, thus substantially differing from thecommonly known steels. M is a standard metallurgical expression definedas the tempera ture at which the martensitic phase begins to form whenthe temperature is lowered. M is a metallurgical term defined as thetemperature above which the martensitic phase cannot form duringmechanical Working of the metal. The M and M may readily be determinedby tests familiar to those skilled in the art. An M of ambienttemperature or below is extremely low compared to the M of commonlyknown heat treatable high strength steels.

It is also essential to the invention that the steel contains at least0.5 preferably about several percent of at least one alloying elementselected from the group consisting of molybdenum, manganese, vanadium,niobium, tantalum, and tungsten, preferably molybdenum.

In addition, the inventive steel must contain chromium in the range fromabout 11% to about 18%, but preferably between about 11-13%.

ice

The steel used in the starting material in the present process, as inthe above referenced patented process, must have a total carbon plusnitrogen content of from about 0.15% to about 0.5%. Since the carboncontent in conventional stainless steels is generally kept below 0.1%,the combined carbon and nitrogen content of the inventive steel is asharp contrast to these commonly known stainless steels.

In the inventive process, if the combined carbon and nitrogen content ofthe steel is below about 0.2% the steel does not have high strength andhigh toughness, and when the combined carbon and nitrogen content of thesteel is above about 0.5% the steel does not have good elongation.

Also, to be suitable for use as a starting material in the presentinvention, the steel, like in the patented process, must first beconverted to a single phase, austenitic state, this being usually doneby heating the steel above the critical temperature for austeniteformation for a length of time sutficient so that the entire specimenhas been heated throughout, as known in the art. In general, for thesteels of the present invention, the temperature will be from about 1500 F. to about 2200 F.

The actual operation to produce the inventive corrosion resistant, highstrength, high toughness, high uniform form elongation steel involvesdeforming a steel specimen having the above described composition withina particular temperature range with the lower limit being about 400 F.and the uppper limit being the recrystallization temperature of thesteel. Recrystallization temperature is a standard term customarilydefined as that temperature above which a completely new set ofstrain-free grain are formed, and, as known in the art, may bedetermined by routine tests. The recrystallization temperatures of thesteels of the inventive steels are generally in the range of about 1500"F. to about 1800 F., and thus the temperature range at which deformationoccurs in the inventive steel is generally from about 400 F. to 1100"F., this being an unusual working range when compared with thetemperature utilized for the commonly known high strength steels.However, some steels made in accordance with the invention may have ahigher recrystallization temperature up to 2200 F. and a deformationtemperature up to about 1800 F. The expression deforming" is used in thedescription of the present invention to mean subjecting the specimen toa stress beyond its elastic limit and thereby causing a change in theshape of the entire specimen. Deformation may be carried out by any ofthe standard metal working techniques such as rolling, swaging, wiredrawing, forging, shear forming, etc. Any application of mechanicalforce sufficient to cause a change in shape is effective provided thechange in shape extends throughout the entire specimen. In general it ispreferred that the amount of deformation be at least about 20% attemperatures in the middle of the operating range. At the lower end ofthe temperature range a greater amount of deformation may be required toinduce the desired chemical and structural changes, while at the upperend of the temperature range a greater amount of deformation may berequired because the higher temperatures may serve to modify thestructure in an undesired way.

The benefits of the process are achieved once the steel of the propercomposition has been deformed at the proper temperature. Afterdeformation, the specimen may be treated in a variety of ways dependingon the ultimate use. For example, the steel may be quenched rapidly toroom temperature in an appropriate quenching medium, and, if desired,even subsequently cooled to a temperature below room temperature.Alternately, it may be quenched rapidly to the desired processingtemperature.

While the high strength, high elongation steel of the above referencepatent contained, for example, about 8% chromium, the present inventionutilizes a chromium content of from about 11 to 18 percent, preferablyabout 1113%, thereby providing good corrosion resistance, whereby notonly does the present invention provide a steel simultaneouslypossessing high strength, high toughness and high elongation butprovides the additional advantage of corrosion resistance, therebysubstantially improving the steel of the above referenced patent.

As in the above mentioned patented steel process, the steel of thepresent invention provides increased elongation resulting directly fromthe control of the austenite to martensite transformation, whileproviding high strength and corrosion resistance.

Therefore, it is an object of the invention to provide a corrosionresistant metastable austenitic steel and process therefor.

A further object of the invention is to provide a high strength, highelongation, corrosion resistance steel.

Another object of the invention is to provide a high strength, highelongation steel having a total carbon plus nitrogen content of from0.15% to about 0.5%, and a chromium content in the range from about 11%to about 18%.

Another object of the invention is to provide a process for producing acorrosion resistant steel possessing both high strength and highelongation, wherein the steel is composed of about 12 to 14% cromium,about to 11% nickel, about 0.2 to 0.5% carbon, up to about 4%molybdenum, up to about 2% manganese, with minor amounts of silicon,phosphorus and sulfur, and the balance iron.

Other objections of the invention, not specifically set forth above willbecome readily apparent from the following description.

DESCRIPTION OF THE INVENTION Recent developments have shown that highstrength and ductility can be obtained by subjecting metastableaustenitic steels to proper combinations of heat treatments anddeformation processes (thermomechanical processing). When thesematerials are strained, they transform from face-centered cubicaustenite to body-centered tetragonal martensite. If this transformationbegins before necking (local plastic instability) occurs, a high rate ofstrain hardening results and the material can continue to deformplastically at high stress levels. Metastable austenitic alloys thatexhibit this type of behavior have been called TRIP (EransformationInduced Elasticity) steels, such steels being disclosed and claimed inthe above-identified patent.

In order for the material to undergo this austenite-tomartensitetransformation, its chemical composition must be properly balanced. TheM temperature must be below room temperature (or below the testtemperature) and the M temperature must be above test temperature. The Mtemperature is always above the M temperature. The M and M temperaturesare influenced both by chemical composition and thermo-mechanicalprocessing. It was commonly held that after thermomechanical processingthe M and M temperatures increase was due at least in part to some ofthe carbon being precipitated during processing to form finely dispersedcarbides, this decreasing the stability of the surrounding austenite.Recently, however, the possibility of the M temperature being decreased,while simultaneously the M temperature is increased by thermomechanicalprocessing, has been determined, as reported by the coinventor of thisapplication, Dr. V. F. Zackay in a University of California reportUCRL-l8676 entitled Anticipated Developments in Physical MetallurgicalResearch, January 1969, see page 16.

It has been found that metastable austentic steels (TRIP steel), asexemplified by the above-identified patent,

undergo an active to passive corrosion behavior similar to stainlesssteels. For example, corrosion tests on a typical TRIP steel, with anominal composition, of 9% chromium (Cr), 8% nickel (Ni), 4% molybdenum(Mo), 2% manganese (Mn), 1% silicon (Si), and 0.25% carbon (C), Balanceiron (Fe), have shown a corrosion rate 2 to 3 times greater than an 18%Cr- 8% Ni austenitic stainless steel in 10% sulfuric acid at roomtemperature.

The present invention provides a solution to this corrosion problem inthe so-called TRIP steels, thereby providing a steel having both highstrength and high elongation which is corrosion resistant as compared tothe TRIP steels of the above-mentioned patent.

In accordance with the invention, it has been determined that goodcorrosion resistance in 10% sulfuric acid is obtained by increasing tochromium content to between 11% to 18%, with 11% to 13% beingpreferable. It has also been determined that molybdenum has a beneficialeffort on the corrosion resistance of the TRIP steel alloy composition,Whereas manganese above a certain amount is detrimental thereto.

The same elements that are present in common stainless steels, i.e., Cr,Ni, and Mo also influence the stability of the austenite-to-martensitetransformation of the TRIP steels. When increasing the Cr content toobtain corrosion resistance in the TRIP steel, the amount of Ni, Mo, Mnand C must be adjusted to give the desired metastable austeniticstructure after thermomechanical processing.

As pointed out above, it is essential that the steel contains at least0.5%, preferably about several percent, of at least one alloying elementselected from the group consisting of molybdenum, manganese, vanadium,niobium, tantalum, and tungsten, preferably molybdenum, in addition tothe chromium content of preferably about 11- 13 The steel used as thestarting material in the present process must have a total carbon plusnitrogen content of from about 0.15% to about 0.5%. To be suitable foruse as a starting material in the operation of the present process, thesteel must first be converted to a single phase, austenitic state. Theusual way of doing this is by heating the steel above the criticaltemperature for austenite formation for a length of time sufficient sothat the entire specimen has been heated throughout. Those skilled inthe art are familiar with this operation and accordingly such need notbe described in detail. In general, for the steels of the presentinvention, the temperature will be from about 1800 F. to about 2200" F.

The actual operation of the process involves deforming a steel specimenhaving the proper composition, for example constituting alloy F of TableI set forth below and composed of 12.3% chromium, 7.8% nickel, 3%molybdenum, 0.24% carbon, with not over 1% silicon, 0.045% phosphorus,0.03% sulfur and with the balance iron. This deformation must take placewithin a particular temperature range. The lower limit of thetemperature range is about 400 F. with the upper limit of thetemperature range being the recrystallization temperature of the steel,generally, in this process, in the range of about 1500 F. to about 1800F., thus the temperature at which deformation occurs is generally in therange of 400 F. to 1100 F. in practicing this invention.

Again, it is emphasized that expression deforming as used herein isintended to mean the subjecting of the specimen to a stress beyond itselastic limit and thereby causing a change in the shape of the entirespecimen, and can be carried out by any of the standard metal workingtechniques such as rolling, swaging, wire drawing, forging shear formingetc., any application of mechanical force being sufficient provided itcauses a change in shape which extends throughout the entire specimen.In general it is preferred that the amount of deformation be at leastabout 20% at temperatures in the middle of the operating range. At thelower end of the temperature range a greater amount of deformation maybe required to induce the desired chemical and structural changes, whileat the upper end of the temperature range a greater amount ofdeformation may be required because the higher temperatures may serve tomodify the structure in an undesired way.

The above exemplary steel composition, processed in accordance with theTRIP steel concept may be carried out as follows: Heat to 2080" F. andhold at this temperature for one hour; cool to 840 F. and deform 80% byrolling at this temperature; finally, water quench to room temperature.The above processing as shown for alloy F in Table II hereinafterresults in the following properties: Yield strength: 187,000 p.s.i.;ultimate tensile strength: 231,000 p.-s.i.; tensile strength to yieldstrength ratio: 1.24; percent elongation of 1 inch: 38; percentreduction in area: 3-8; R hardness: before: 45, after: 57; magneticcharacteristics; before: non-magnetic, after: magnetic.

To illustrate the advantages of the present invention, a comparison ofthe mechanical properties as set forth in Table II between the abovenovel steel composition and those of the commercial type 316 stainlesssteel, the composition of which is set forth in Table I below, will showthe corrosion resistant, high strength, high elongation steel of thepresent invention as having about 5.3 times greater yield strength andabout 2.7 times greater tensile strength than the type 316 steel.

TABLE I.OHEMIOAL COMPOSITION OF ALLOYS IN WEIGHT PERCENT Cr Ni Mo Mn 0Fe This alloy also contains a maximum of 1.00% silicon, 0.045%phosphorus and 0.030% sulfur.

TABLE II.-MECHANIOAL PROPERTIES Magnetic R hardness characteristic b Tile Percent Percent strength elong. red. in Before After Before After Ks.i. T.S.IY.S. 1 inch area test a test b test test 253 1.54 28 32 48 55Non-mag. Mag. 187 1.0 8 51 41 41 Sl-mag. Mags: 194 1.0 11 51 42 42Sl-mag. Mag 209 1.05 46 44 45 55 Non-mag. Mag. 264 1.39 27 48 56 Sl-mag.Mag. 231 1.24 38 38 45 57 Non-mag. Mag. 249 1.35 34 42 49 58 Non-mag.Mag. 231 1.25 33 44 57 Non-mag. Mag. 188 1.01 46 42 44 52 Sl-mag. Mag.

85 2. 42 f 55 65 80 Non-mag. 85 2. 42 f 55 70 80 Non-mag.

1 2 inch gage length.

The inventive steel composition may, for example, also be produced bythe following treatment: heat to 2080 F. and hold for one hour at thistemperature, water quench to room temperature, reheat to 840 F. anddeform 80% by rolling at this temperature. Alternately, the specimen canbe cooled from 2080 F. to 840 F. rather than to room temperature andreheated to 840 F.

The inventive steel of the above composition, for example, may also beprocessed by the following treatment: heat to 2080 F. and hold for onehour at this temperature, water quench to room temperature, heat to 840F. and deform 80% by rolling at this temperature, cool to roomtemperature, strain 15% at room temperature, and finally, heat to 840 F.and hold at this temperature for thirty minutes.

To verify the inventive concept as well as to clearly illustrate that asteel may possess both high strength and high elongation and becorrosion resistant at least to certain corrosion type environments,extensive testing has been conducted to determine the effects producedby variations in the TRIP alloy composition. As a result of these teststhe inventive concept has clearly been supported resulting in optimizinga steel with corrosion resistance and with the TRIP steel mechanicalproperties. Since the details of those tests do not constitute part ofthis invention, only a brief description thereof will be set forth.These tests were conducted under the supervision of the inventors of thepresent invention, and the procedures and results of these tests arefully described in Report UCRL-19065 entitled Optimization of CorrosionResistance in Metastable Austenitic Steel" authored by In the tests thealloy processing was accomplished by producing heats of ingots byinduction melting of high purity elements in a helium; forging theingots at 1100 C. into fiat bars; cross-rolling the flat bars at 1100 C.to a thickness of 0.4 inch; austenitizing the material at 1200 C. for 2/2 hours in Sentry-Par: stainless steel bags to preventdecarburization', and quenching in an ice-brine solution. Thethermomechanical treatment consisted of an reduction in thickness byrolling at 450 C., the material being reheated to 450 C. after each passof 10 to 15 mils until a thickness of 0.08 inch was reached, after whichit was water quenched.

The mechanical testing was accomplished by machining tensile specimensfrom the 0.08 inch rolled materials, with 1 inch gage length specimensbeing used. The tensile tests were carried out at room temperature usinga cross-heat speed of 0.04 inch/min. Rockwell C hardness measurementswere made on the tensile specimens before tensile testing outside of thegage length, and after the tensile test the hardness measurements weremade within the gage length. Magnetic measurements were made along thegage length of each specimen before and after the tensile test by usinga large hand magnet.

An electrochemical method of corrosion testing known as thepotentiodynamic polarization technique was used in evaluating thecorrosion resistance of the inventive alloy. The importance of thismethod is that it allows a series of alloys, exhibiting passivity, to besystematically tested for their corrosion properties in a relativelyshort time (3-4 hours). Briefly, a typical potentiodynamic anodicpolarization curve was prepared, which is a plot of the potential of themetal in an electrolyte (against a standard such as a calomel electrode)vs. the current density developed by the metal at this potential (with asuitable auxiliary electrode such as platinum), the curve beingdeveloped by slowly, but continuously varying, the potential. As known,the curve along the potential axis can be divided into three parts,i.e., (1) active, (2) passive, and (3) transpassive regions. From thiscurve it is possible to determine the following:

(1) The mixed or corrosive potential (E The potential of the metal inthe environment without any current flowing, i.e., with no dissolutiontaking place.

(2) The primary passive potential (E Up to the primary passivepotential, normal dissolution of the material takes place. As thepotential is made more noble, up to the primary passive potential,dissolution of the material increases linearly, a behavior typical ofall nonpassivating materials. At the primary passive potential thematerial begins to exhibit a decrease in the dissolution rate(passivation), i.e., the current density decreases. This decrease isthought to be due to the formation of a protective film on the metalsurface. The corrosion resistance of a material may be effectivelyincreased by adding alloying elements which shift the primary passivepotential in the active direction, since, for chemical passivation totake place, the environment (oxidizing agent) must have a higher (morenoble) redox potential than the primary passivation potential of themetal. Hence, the lower (more active) the primary passivation potentialof the metal, the greater its corrosion resistance, even in the presenceof a weak oxidizing agent, i.e., one that has a low redox potential. Insteels, the usual alloying additions of chromium, nickel, and molybdenumgenerally have only a minor effect on the primary passive potential.Hence, a more important parameter in improving corrosion resistance isthe critical current density set forth herebelow.

3) The critical anodic current density (1 The maximum corrosion ordissolution rate fo the metal in the environment, and indicates thecurrent that must be achieved in order for the material to passivate.The lower the value of the critical current density, the lower theconcentration of oxidizing agents necessary for achieving passivity.Hence, for increased corrosion resistance, alloying additions whichlower the critical current density of a material are desirable. Thislowering of I is generally more effective in increasing passivatingtendency than is the altering of the primary passivation potential. Thereason for this is that in practical situations, the oxidizingenvironment (usually containing oxygen) has a redox potential higherthan the primary passivation potential. Therefore, the main barrier topassivation becomes the ability of the environment, by its reduction onthe metal, to produce a current density greater than the critical anodiccurrent density.

(4) The passive potential region: Indicated by the vertical, low,constant-current-density part of a typical potentiodynamic curve. From acorrosion viewpoint the passive area should be as wide as possible. Thematerial can remain passive under more varied conditions when theprimary passive potential (E described above, and the transpassivepotential (E,,) are farther apart.

(5) The passive current density (1 Given by the constant value of thecurrent density in the passive range and indicates the passive corrosionrate. This represents the lowest amount of corrosion taking place. Thecurrent density can be related to the corrosion rate by Faradays Law.Taking into account the percentage of each element in the alloy, anaverage value of 0.5 mil per year being equivalent to 1 arnp/cm. isobtained. Since the rate of corrosion in the passive state isproportional to the passive current density, increased corrosionresistance can be obtained by lowering the passive current density.Also, the lower the value of I the more stable the passive statebecomes. The reason for this is that the lower the passive currentdensity becomes, the less current that must be supplied by thepassivating (oxidizing) agent in order to re-establish a passive filmthat has been temporarily destroyed.

(6) The transpassive area: In the transpassive area the passive state isdestroyed and the current density begins to increase again similar tothat in the active region. In some steels a secondary passivity isfrequently observed. From a practical standpoint, this secondarypassivity is not important due to its small size and instability. Alsosteels cannot normally reach this high potential except when a externalcurrent is supplied. This secondary passivity has been ascribed to theadsorption of oxygen at the potential which is just before evolution ofoxygen in gaseous form occurs.

It is thus seen that the above items 1-6 serve as a guide showing how ananodic polarization curve was used to systematically follow the effectof composition or other variables on the corrosion properties of thealloys tested. It should be noted also that cathodic reduction of theparticular alloy has notable influence in the corrosion system, thisbeing well known in the art.

Since the details of the experimental test technique do not constitutepart of this invention, greater description thereof is deemedunnecessary.

In the tests conducted to verify the inventive corrosion resistant, highstrength, high uniform elongation steel, the only variable in all of thealloys was the chemical composition. All other processing and testingfactors were held constant, i.e., austenitizing temperature (2080 F.),deformation temperature (840 F.), amount of deformation and testingtemperature (72 F.). The mechanical properties of the alloys indicatethat the resulting structures ranged from metastable to completelystable austenite. A summary of these properties for alloys A-I is givenin Table II, above, along with values from the literature for type 304and type 316 stainless steel. Alloys A, E, F, G, and H were sufiicicntlymetastable to undergo the TRIP steel phenomenon, and excellentstrengthductility values were obtained. The austenite in alloys D and Iwas metastable and underwent the austenite to martensite transformation,but at an insuflicient rate to show appreciable strain-hardening (TS/YSratios of 1.05 and 1.01, respectively).

Alloys A, D, F, G, and H were assumed to be completely austenitic beforetesting as indicated by their nonmagnetic behavior. The remaining alloyswere slightly magnetic before testing, indicating a small amount ofmartensite was present.

All of the alloys which underwent the austenite to martensitetransformation exhibited a sharp yield point and Luders strain. Afterthe Luders strain, the slope of the stress-strain curve (not shown)increased and a number of serrations appeared. These serrations havebeen attributed to the formation of martensite in local necked regionsof the specimen. Due to the higher strain in these regions, martensiteforms and strengthens this region against further necking, this beingdue to the TRIP phenomenon. Martensite is a more effective barrier todislocation motion, hence deformation then proceeds in other areas ofthe specimen. These serrations may also be due to thePortevin-LeChatelier effect, but such effect probably plays only a minorrole in these alloys due to the low diffusion rate of the soluteelements at room temperature.

The difference between alloys A and B, see Table II, was that alloy Bcontained 2.7% more Ni, see Table 1. Alloy B has a higher yield point,probably because of a combination of solution hardening and the factthat a small amount of martensite was initially present in alloy B. Thismartensite may have been produced either by the quenching afteraustenitizing or during deformation of the austenite at 450 C. Since thematerial behaved in a somewhat ductile manner, showing cup-cone fractureand 8% elongation, its non-TRIP behavior was probably due to thestability of the austenite against transformation by straining, and notto the initial martensite present. The influence of composition alone onthe stability of the austenite in alloy -B is indicated by its higherposition in a Modified Schaefiler Diagram, not shown but widely utilizedin the art.

Comparing alloys B, C, D, F, and G (see Tables I and II) it is seen thatas M is added and Ni decreased,

corrosion potential (E of the alloys was indicated on the polarizationcurves, not shown. The passive potential region for all alloys testedranged from approximately 0.1 v. to 0.95 v. The passive potential regionwas slfightly larger for the alloys containing greater amounts 0 M0.

The influence of nickel (Ni) on the inventive steel is seen by comparingalloys A and B in Table III below:

TABLE III.SUMMARY OF ANODIC POLARIZATION RESULTS AND MECHANICALPROPERTIES l Weight percent 5 I I d tYielrgh tTensile Percent b vs. p sreng s ren th elon Alloy Cr N1 M0 Mn 0 Fe son. a/cm 2 ta/c111 K 5.1. Ksj. 1 in A 12.9 7.8 0.34 960 11 164 253 28 B 13.0 10.5 0. 32 720 9.5 187187 8 C 13.0 10.0 0. 31 98 11 194 194 11 D 12.6 8.8 -0. 27 35 12 200 20946 E 13.4 7.6 0. 32 76 13 190 264 27 F 12.3 7.8 0 24 17 7 187 231 38 G12. 9 6. 9 0. 25 12 9. 5 185 249 34 H 13. 0 5. 9 0. 28 30 8. 5 185 23140 I 12.8 5.8 0.38 38 9.5 186 188 46 304 18. 7 9. 1 0. 22 84 4 85 65 31618.0 13.5 0. 18 16 4 35 85 55 a Mechanical properties for type 304 andty e 316 taken from the Metals Handbook, 1961. b E =Primary passivepotential. 0 I.,,=Critical anodic current density.

d I =Passive corrosion current density (1 amp/em. =0.5 mil per year).

I 0.2% ofiset yield strength for type 304 and type 316 stainless steels.

2inch gage length for type 304and type 316 stainless steels.

I T1118 alloy also contains a maximum of 1.00% silicon, 0.045%phosphorous and 0.030% sulfur.

a more unstable austenite is produced. This leads to excellentstrength-ductility values as a result of the strengthening action of theaustenite-to-martensite transformaion. The increase in yield strength ofalloys B, C, and D may be due to a combination of solution-hardening bythe Mo, and hardening by precipitates of molybdenum carbides. At a Mo/Niratio of 3/8 (alloy F) the yield strength drops, and at a ratio of 4/7(alloy G) the yield strength slightly decreases again. This indicatesthat the beneficial efforts of solution and precipitation hardening mayhave been reached.

Comparing alloys F and H it is seen that Mn can be directly substitutedfor Ni. The two alloys have approximately the same mechanicalproperties.

While the tests did not establish a definite relationship between theposition of the alloy in the Modified Schaeifier Diagram, its calculatedM temperature, and its mechanical properties, it was observed that ifthe position of the alloy was too far above the austenitemartensiteboundary the TRIP phenomenon did not occur. Also, while the Modified'Schaeffler Diagram and the calculated M temperature serve as guides inselecting compositions that may undergo the transformation fromaustenite to martensite after yielding of the material takes place,these do not take into consideration the effect that thethermomechanical processing has on M and M and hence on the resultingmechanical properties.

The effects of Ni, Mo, and Mn on the electrochemical parameters aresummarized in Table III below. For comparison, the electrochemicalvalues obtained for type 304 and type 316 stainless steel are alsolisted in Table III.

The electrochemical parameters listed in Table III are the primarypassive potential (E the critical anodic current density (1 and thepassive corrosion current density (I The effect of the alloying elementswas most pronounced in the active region, where significant changes wereobserved in the critical current density (1 and the correspondingpotential (E Current density in the passive state (I was approximatelythe same for all the materials tested and ranged from a minimum of 7a./cm. (Alloy F) to a maximum of 13 a./cm. (Alloy E). On the other hand,the I ranged from a low value of 12 a/cm. (Alloy G) to a high value of'960 ,ua./cm. (Alloy A). The mixed or natural As seen, the extra 2.7% Niin alloy B decreased I from 960 ,ca./cm. to 720 ,ua./cm. and I from 11,na./cm. to 9.5 ,ua./cm. As expected E and E became slightly morepositive. Regarding the column E in Table III term (V vs. S.C.E.)defines the potential of volts vs. a saturated calomel electrode. Asalso seen, the eifect of Ni on the mechanical properties of the alloysis significant as discussed above.

The influence of the No/Ni ratio can be seen by comparing alloys B, C,D, F, and G, the nominal ratios being 7 0/11, 1/10, 2/9, 3/8, and 4/7.The M0 content was increased by l% steps as the Ni content was decreasedby a similar amount. As can be seen from Table HI, there is a tendencytoward more positive primary passive potentials (E as the M0 isincreased and Ni decreased. The outstanding effect that Mo has is indecreasing the critical anodic current density (I Comparing alloys B andC, 1t is seen that a very large decrease in I is obtained by thesimultaneous decrease of Ni by 1% and increase of M0 by 1%. I isdecreased by at least half of the previous value in going from a Mo/Niratio of 1/ 10 to 2/9 and from 2/9 to 3/8. When this ratio is changedfrom 3/ 8 to 4/7, I only decreases by one-third, thus indicating thebeginning of a leveling effect. The extent of the passive range alsoappears to have been maximized at a No/Ni ratio of 4/7. Molybdenum hasbeen used to decrease the susceptibility of stainless steel to pittingby causing a more protective or more stable passive surface, and thusindicates from the test conducted that alloys F and G, which have thelowest I may have a high resistance to pitting. The passive currentdensity (I of alloys C (ll/l0) and D (2/9) increased slightly over thatof alloy B (0/11). Alloy F with a Mo/Ni ratio of 3/8 showed a decreaseof I to a value of 7 na./cm. This was the lowest passive current densityobtained for all the alloys tested. This corresponds to a corrosion rateof approximately 3.5 mils per year in the passive range. Examination ofTable III shows that with an increase of the Mo/Ni ratio of 4/7 (alloyG) ,an increase in I to the value of alloy B (0/11) takes place, whichappears to indicate that the beneficial effect of Mo/Ni is best at aratio of 3/8 (alloy F).

The influence of Mo on alloys A-I with a fixed Ni content of 8% can beseen by comparing alloys A, E, and F in Table III wherein the values ofE I and I are given. As can be seen in Table III, a 1% addition of Moresulted in a very large decrease in I The addition of Mo to 3% caused afurther reduction of I to a value of 17 ,ua./cm. The value of 1;,actually increased slightly with the addition of 1% Mo, but decreased tothe lowest value of all alloys tested when the Mo was increased to 3%.Thus, there is an increase in the stable passive potential range foralloy F over alloys A and E. Alloy F compares very favorable from acorrosion resistance standpoint with both types of stainless steelstested, as can be seen in Table III. The value of I, was much lower foralloy F than for type 304 stainless steel. This indicates that a muchlower current (or lower concentration of oxidizing agent) is required inorder to achieve passivity with alloy F or type 304 stainless steel.Once passivity is achieved, type 304 and type 316 stainless steels willcorrode at a slightly lower rate than alloy F. The lower value of I forthe stainless steels as compared to alloy F is due to their higherchromium content. All of the alloys tested, including type 304 and type316 stainless steels indicate that their passive corrosion rate is wellunder m.p.y., the maximum corrosion rate which would enable a materialto be used in a process with only minor maintenance. Actual long termcorrosion studies show that type 316 stainless steel corrodes at a rateless than 20 mils per year in an air-free, 10% sulfuric acid solution atroom temperature. The test conducted indicate that alloy F, being verysimilar to the test results of type 316 stainless steel, may havesimilar corrosion resistance under the same environment, althoughverification has not yet been accomplished.

The influence of Mn on alloys A-I can be seen by comparing alloys H, Iand F in Table III. Alloys H and I were identical in composition exceptfor the Mn. The major effect of Mn was increasing the critical currentdensity (lot), and hence decreasing the stability and ease of achievingpassivity. Comparison of alloy H with alloy P, which has the same nickelequivalent, i.e., austenite forming elements (Ni, Mn, C) and the samemechanical properties, shows that I is approximately double by theaddition of 2% Mn. Addition of Mn up to 4% (alloy I) caused a furtherincrease in I but due to the lower carbon content of this alloy, theincrease is not very large. The passive current density (I alsoincreased slightly over those alloys without Mn. The potential range ofthe passive region was not greatly affected by the Mn additions.

Comparing alloy H (2% Mn) with type 304 stainless steel (Table HI) showsthat, even with the manganese addition, the critical current density (Iof alloy H remains twice as low as for type 304 stainless steel.

As illustrated above, the elfects of composition on the mechanical andcorrosion properties of the inventive metastable austenitic steels havebeen determined, a summary of these properties being found in Table III.The base composition range for this series was 12.3%13% Cr and0.185%-0.26% C with the balance being iron, different amounts of Ni, Mo,and Mn being added to vary the composition.

In view of the foregoing, it has been shown that the present inventionprovides a metastable austenitic steel with greatly improved corrosionresistance but with similar mechanical properties of the previouslyknown TRIP steels disclosed in the above referenced US. Pat. No.3,488,231. The inventive concept has clearly been illustrated by testswhich support the invention as producing a substantial step in the stateof the art by providing a high strength, high elongation, corrosionresistant steel.

While particular compositions and operational sequences have been setforth to describe the inventive concept, modification and changes willbecome apparent to those skilled in the art, and it is intended to coverin the appended claims all such modifications and changes as come withinthe spirit and scope of the invention.

What We claim is:

1. In a process for producing a substantially austenitic steel having acombination of high strength, high uniform elongation, high toughness,and corrosion resistance by utilizing a strain-induced transformationwhich occurs in service, the steps of subjecting single phase,austenitic steel which has an M below ambient temperature, a totalcarbon plus nitrogen content of from about 0.15% to about 0.5%, achromium content of from about 11% to about 18%, and which contains atleast 0.5% of at least one alloying element selected from the groupconsisting of molybdenum, manganese, vandium, niobium tantalum, andtungsten, to deformation at a temperature above the M temperature, butbelow the recrystallization temperature of the steel, while maintainingsame in substantially austenitic form, and cooling to ambienttemperature while maintaining same in substantially austenitic form.

2. The process defined in claim 1, additionally, including the steps offorming the thus composed austenitic steel prior to the deformationthereof by heating the thus composed material thereof to a temperatureabove the critical austenite formation temperature thereof for a periodof time sufiicient to assure transforamtion of substantially all of thethus composed material to the austenitic phase, and bringing thetemperature of the thus formed austenite steel to the temperature ofdeformation.

3. The process defined in claim 2, wherein the step of bringing thetemperature of the thus formed austenitic steel to the temperature ofdeformation is accomplished by quenching the thus heated austeniticsteel to room temperature and reheating the thus quenched austeniticsteel to the deformation temperature.

'4. The process defined in claim 2, wherein the step of bringing thetemperature of thus formed austenitic steel to the temperature ofdeformation is accomplished by quenching the thus heated austeniticsteel to the deformation temperature.

5. The process defined in claim 2, wherein the temperature above thecritical austenite formation temperature is from about 1500 F. to about2200 F., and wherein the temperature of deformation is from about 400 F.to about 1800 F.

6. The process defined in claim 2, wherein the step of forming the thuscomposed austenitic steel includes preparing the thus composed materialto consist essentially of a composition of about 12.3% chromium, about7.8% nickel, about 3% molybdenum, about 0.24% carbon-nitrogen, and thebalance iron, and wherein the temperature above the critical austeniteformation temperature is about 2080 F. with the time period being aboutone hour, wherein the temperature of deformation is about 840 F., andwherein the deformation of the austenitic steel is in the range of about20% to about 7. The process defined in claim 1, additionally includingthe step of subjecting the thus formed substantially austenitic steel tostrain thus inducing transformation of the substantially austeniticsteel to martensitic steel, whereby the strength of the steel isincreased.

8. The process defined in claim 1, wherein the single phase, austeniticsteel has a carbon plus nitrogen content in the range of about 0.185% toabout 0.26%, a chromium content in the range of about 12% to about13.4%, a molybdenum content in the range of about 1% and about 4%, withthe balance being essentially iron, and wherein the temperature abovethe critical austenite formation is about 2080 F. with the time periodbeing about one hour, wherein the temperature of deformation is about840 F., and wherein the deformation of the austentic steel is in therange of about 20% to about 80%.

. 9. The process defined in claim 8, additionally including the steps ofcooling the thus deformed austenitic steel to room temperature,straining the thus cooled steel about 15% at room temperature, andheating the thus straining iteel to about 840 F. for a time period ofabout one-half our.

(References on following page) 13 14 References Cited 3,240,634 3/1966NachEman 148-12 UNITED STATES PATENTS 3,281,287 10/1966 Edstrom et a1148-12 3,425,377 2 1969 Deacon 24 WAYLAND STALLARD, Primary Examin r3,488,231 1/1970 Zackay et a1. 14s -12 5 3,216,868 11/1965 Nachtman148-12 148 12 4

